The present invention is directed to telephone equipment and in particular to call-disposition analyzers.
Many offices, hotels, and so forth have private branch exchanges (PBXs) for making connections between telephones at the local premises and trunk lines that connect the local premises to the public telephone network. Such equipment primarily performs switching functions, but many types additionally generate data, in the form of station message detail records ("SMDR records"), that can be used by call-accounting and other systems to keep track of various aspects of the telephone traffic that the PBX handles.
In the case of a hotel, one of the ways to which the information contained in the SMDR records is put to use is to enable the hotel to recover the cost of its customers' toll calls. Unfortunately, although the SMDR records do report PBX usage quite effectively, they are not typically arranged to indicate whether a given use resulted in a successful telephone call. As a result, the hotel may charge a guest for calls that were attempted but not completed. This affects guest relations adversely. Conversely, by treating all usages of less than, say, forty-five seconds as unanswered calls, the hotel can fail to obtain reimbursement for a call for which it is required to pay.
The obvious solution to this problem, of course, is to provide the PBX equipment with circuitry for determining whether the calls are completed. The PBX could then include the resulting information in the SMDR records. But most PBX users do not need this added capability and do not want to pay for it, so most PBX equipment does not include it. Moreover, most hotels have already made a considerable investment in their PBX equipment and would therefore be reluctant to replace it even to obtain the extra capability.
To deal with this problem, telephone-equipment manufacturers have produced "call-disposition analyzers" or "answer supervision devices," which can be installed on existing PBXs. A call-disposition analyzer monitors a PBX's trunk lines as well as the lines by which the PBX sends SMDR records to the hotel's call-accounting system. From the signals thus observed, the call-disposition analyzer infers the "disposition" of the call--i.e., it infers (at least) whether the call was completed. For instance, the call-disposition analyzer may monitor a given trunk line to detect various sequences of dial, supervisory, and voice signals that are symptomatic of call completion. Interposed between the PBX and the call-accounting system, it may then withhold the next SMDR record that designates that trunk line if the call-completion-identifying sequence did not appear; i.e., the call-accounting system may receive records only for completed calls.
This approach is quite effective, and it enables the hotel to recover its toll-call costs effectively without improperly charging guests. Unfortunately, the call-disposition analyzers currently available are somewhat elaborate and thus expensive, and they tend to be justified for only the largest hotels, i.e., those that have more than, say, two hundred rooms. Most hotels are smaller than this.
A major part of the reason for their relatively high cost is the complexity of the circuitry and programming required to infer call completion. This complexity results largely from the lack of uniformity among the SMDR formats used by the various types of PBX systems currently in use. A commercial call-disposition analyzer must be manufactured to include programming that makes it compatible with all or most of the types of PBXs on which it might be installed, and, when it actually is installed, it must be informed of the particular type of PBX with which it is to operate. Even these measures are not always adequate, moreover, since PBX manufacturers are not unfailingly so obliging as to notify call-disposition-analyzer manufacturer of SMDR-format changes.
Another contributor to call-disposition-analyzer cost is the need to monitor the trunk lines by which the PBXs are connected to the telephone network. Lines connected to the telephone network often are highly susceptible to noise. This noise usually does not detract significantly from actual telephone use; it contains mostly common-mode components, while the telephones operate on difference-mode signals. But the common-mode noise often results in significant voltages between the public-network ground and the monitoring-equipment ground, and such voltages can drive trunk-line-monitoring voltage sensors to distortion if measures are not taken that typically add to the system cost.